For many paper mills, the main fiber source to waste water treatment comes from rejects of Dissolved Air Flotation (DAF) systems located on the paper machines. DAF systems are primarily used on paper machines to filter out very short fibers and fines present in the pulp sources that are not suitable for use on the paper machines. For ultra-premium tissue and towel products, incorporation of these fines and short fibers into the product can densify the sheet which reduces the flexibility and perceived softness of the product, which is undesirable. To remove these short fibers and fines, a portion of the reused process water is continually pumped to the DAF for fiber removal. The process water is re-used to dilute the incoming pulp to a proper consistency for use on the paper machine. Specifically, the process water drains through the paper machine fabrics, and a portion of this process water is then processed at the DAF to remove the fines and short fibers contained in the process water. After processing at the DAF, the water is reused again in a continuous loop to dilute more pulp to be used on the paper machine.
In many paper mills, the fibers removed from the DAF are sent to the process sewer which flows to waste water treatment. These fibers make up a large percentage of the biological oxygen demand (BOD) which must be reduced to a permissible level before the water can be discharged back into the environment. At waste water treatment, there are many ways to remove or degrade the fibers. Many treatment plants can remove a portion of the fibers using a clarifier, screw press, or large DAF system. These fibers are then most often sent to a landfill, or dewatered and burned at the plant to produce energy. With much of the fiber now removed, the remainder of the fibers and other BOD contributing components, such as chemicals, are degraded using microbes in Aeration Sedimentation Ponds or similar types of processes. Once treatment is complete, and the BOD level is at the permitted level, the water can be discharged back to the environment.
Waste water treatment costs can be significant and also restrictive to paper mill production if the water cannot be treated to meet environmental standards including BOD levels. There exists the need to reduce the costs of waste water treatment as well as improve the efficiency of waste water treatment to ensure profitability and operability within a paper mill.